1. Technical Field
This disclosure relates to a solder alloy material layer composition, an electroconductive and adhesive composition, and a flux material forming composition used for forming projected electrodes (the so-called bumps) at bump forming points on an object on which the bumps are to be formed; solder ball transferring sheet; bump forming process for forming the bumps; and semiconductor device with the bumps formed by the bump forming process.
2. Description of the Related Art
Recently, semiconductor packages have been more and more integrated to need more input/output terminals, and becoming more difficult to assemble by lead pins or the like. Therefore, the conventional packaging technology has been replaced by the so-called ball grid array (BGA) technology, which uses solder balls in place of pins for bonding semiconductor chips to a printed-wiring board.
The package type called chip size package (CSP), whose size is decreased as far as possible to a semiconductor chip size, has been developed to realize the package of smaller size and higher density. For the CSPs, the so-called flip chip bonding technology, which bonds semiconductor chips to a board with the pad opening side facing the board, has been attracting attention and extensively studied.
The flip chip bonding technology needs a bump on a chip to bond the chip to a board, and a solder ball is used as the bump in some methods. The method which has been attracting attention for forming the bumps collectively forms a number of solder ball bumps for each semiconductor chip on a silicon wafer before it is diced into individual chips.
The solder bumps may be collectively formed at low cost on electrodes on a silicon wafer by the following known techniques: (1) a creamy solder material is screen-printed on electrodes on a wafer and then fused under heating, or (2) wedge-like cavities are formed in a diamond-shape pattern on a silicon substrate by anisotropic etching, filled with solder paste and heated to form solder balls, which are then transferred to a semiconductor package or semiconductor electrodes (disclosed by, e.g., Japanese Patent Application Laid-Open (JP-A) No. 9-36118).
When bumps are to be formed on an object on which the bumps are to be formed, e.g., wafer with circuits (hereinafter sometimes referred to as “integrated circuits (ICs)”) for corresponding IC chips, they are formed on each of external electrodes (pads) on each IC by the following procedure.
First, the periphery around each pad of aluminum or the like for each IC formed on a wafer on one side, is electrically protected with a passivation film. Next, that wafer side is laminated with a positive or negative photoresist film, which is exposed for each pad, and developed to expose the pad. Then, the wafer is set on a sputtering apparatus, and formed on that wafer side by sputtering is a ball limiting metal (BLM) film layer capable of improving adhesion between the pads on the wafer and bumps.
The BLM film layer comprises an adhesive layer of chromium (Cr) or the like and barrier metal layer (or barrier layer) of copper (Cu) or the like, laminated in sequence. It is referred to as an intermediate metallic layer or Cr/Cu/Au layer.
Then, the photoresist film is separated from the wafer to remove the associated BLM film layer laminated on that photoresist film, leaving the BLM film layer laminated on each pad on the wafer.
Then, that wafer side is again laminated with a photoresist film, which is exposed for each pad, and developed to expose the pad. It is laminated with a bump material by an adequate method, e.g., plating, evaporation or printing (in which a metal mask with openings corresponding to each BLM film layer is used, in place of the photoresist film). Then, the photoresist film laminated on one side of the wafer is separated from the wafer to remove the associated bump material laminated on that photoresist film, leaving the bump material on each BLM film layer.
The bump material comprises layers of lead and tin laminated one over another, where it is prepared in such a way that lead accounts for 90 to 98% by mass of the whole bump material and tin for 10 to 2% by mass depending on the amount of lead.
Then, the wafer is spread on one side with a flux by an adequate procedure, and heated at a given temperature in a fusing furnace in an atmosphere of nitrogen, to fuse lead and tin in the bump material. This synthesizes the solder of lead and tin, producing solder spheres by the surface tension of the melt metal and an effect of the flux. Thus, the spherical bumps of the solder are formed on each pad for the ICs on the wafer via the BLM film layer.
The above procedure for forming the bumps invariably needs the steps for forming a photoresist film and for releasing the film, as discussed above, when a BLM film layer is to be laminated on each pad on a wafer (this step is sometimes referred to as “BLM film layer forming step”). It should be noted that number of wafers which a sputtering apparatus can process in one step is limited to around 3 to 5. Therefore, the BLM film layer forming step involves the problems resulting from the complex steps and very time-consuming works it needs.
Moreover, the photoresist film releasing step removes the photoresist film together with the associated BLM film layer, the separated BLM film layer accounting for at least 99% by mass of the whole BLM film layer laminated on the wafer. As a result, 2 or more types of metallic materials for the BLM film laminated on the wafer on one side are wasted almost totally, pushing up the production cost.
Moreover, chromium for the BLM film layer is a metal harmful to human. Still more, a strongly alkaline solution and organic solvent, also harmful to human, are needed for releasing the photoresist film from the wafer. Therefore, the BLM film layer forming step should secure the safe working environment for the workers, and need some facilities therefor and waste treatment system for safely treating the alkaline solution and organic solvent wastes, making the whole system more complex.
The photoresist film releasing step may not completely remove the film, slightly leaving it on the passivation film. The residual photoresist may be scorched on the passivation film, when each bump material on the BLM film layer is fused under heating, which causes other problems.
On the other hand, the step for forming bumps (hereinafter sometimes referred to as “bump material forming step”) invariably needs the steps for forming a photoresist film (or metal mask) and for releasing the film, when the bump material is to be laminated on the BLM film layer, as is the case with the BLM film layer forming step, making the bump material forming step more complex.
Moreover, the photoresist film releasing step in the bump material forming step removes the photoresist film together with the associated bump material laminated on the wafer on one side, the separated bump material accounting for at least 99% by mass of the whole bump material laminated on the wafer, as is the case with the BLM film layer forming step. As a result, the bump material (composed of lead and tin) laminated on the wafer on one side is wasted almost totally, pushing up the production cost.
Still more, the photoresist releasing step in the bump material forming step may not completely remove the film, slightly leaving it on the passivation film. The residual photoresist may be scorched on the passivation film, when the bump material is fused under heating, which causes other problems.
The plating treatment, when used for the bump material forming step, needs an acidic solution, alkaline solution and organic solvent harmful to human. Therefore, this step should also secure the safe working environment for the workers, and need some facilities therefor and waste treatment system for safely treating the acidic solution, alkaline solution and organic solvent wastes, making the whole system more complex.
The bump material forming step involving plating for the wafer should cope with irregularities caused by the photoresist film formed beforehand on the wafer on one side, because plating current distribution, which depends on wafer surface conditions, may be uneven. Therefore, it may be difficult for the plating treatment to distribute lead and tin which constitute the bump material on the wafer surface to given thicknesses, possibly causing uneven bump material composition.
Therefore, when the resulting bump material formed on the BLM film layer is fused under heating, after a flux is spread thereon, the fused lead will be unevenly synthesized with tin, to cause defects, e.g., bubble-caused defects (hereinafter sometimes referred to as “voids”) or cracks within the bump, and other problems, e.g., formation of bumps with different sizes, formation of distorted bump (hereinafter sometimes referred to as “irregularly shaped bump”) or scattering bump on the passivation film (such a bump is hereinafter sometimes referred to as “scattered bump”). These problems can lead to bridging of the adjacent bumps.
On the other hand, the bump material forming step which uses evaporation involves problems resulting from very time-consuming works for laminating a bump material of the BLM film layers for a plurality of wafers, because number of wafers which an evaporator can process in one step is limited to around 3 to 5. Moreover, it may be difficult for an evaporator to distribute the evaporation sources (lead and tin which constitute the bump material) on the wafer surface to a given thickness, because of structural reasons, e.g., the wafer being set up with its surface slanted.
These problems may scatter the bump material composition of lead and tin, as is the case with the step involving plating. Therefore, when the resulting bump material on the BLM film layer is fused under heating to form the bumps, the fused lead will be unevenly synthesized with tin, to cause voids or cracks within the bump, bumps of irregular size or shape, or scattered bumps on the passivation film. These problems can lead to bridging of the adjacent bumps.
On the other hand, the printing treatment, when used for the bump material forming step, puts a creamy solder by a squeezee on the BLM film layers via a metal mask with openings each corresponding to a BLM film layer. This treatment involves problems of deteriorated printing accuracy resulting from, e.g., uneven gap between the squeezee and metal mask to scatter quantity of the creamy solder printed on the BLM film layers via the openings of the metal mask. As a result, it can deteriorate reliability on printing accuracy. Moreover, the unevenly distributed creamy solder may cause bumps of irregular size or shape, which can lead to bridging of the adjacent bumps.
This bump forming process is designed to wash out the residual flux on the wafer with various organic solvents, after forming the bump on each pad on the wafer. Therefore, this washing system should secure the safe working environment for the workers, and need some facilities therefor and waste treatment system for safely treating these organic solvent wastes, making the system more complex.
As discussed above, the conventional bump forming process involves a plurality of steps, and any step failing to satisfy the desired performance makes all the upstream steps useless. Therefore, in this bump forming process, the damage becomes larger as a failure occurs at a more downstream step.
Some bump forming processes have been proposed to solve the above problems. For example, Japanese Patent Application Laid-Open (JP-A) No. 9-205095 discloses a process for forming bumps at bump forming points on an object on which these bumps are to be formed, comprising the first step for disposing the bumps on a substrate on one side such that each position of the bump corresponds to a bump forming point on the object on which these bumps are to be formed, second step for supplying an electroconductive adhesive agent to the bumps or the bump forming points on the object on which these bumps are to be formed, third step for positioning each bump and corresponding bump forming point by placing the substrate against the object on which these bumps are to be formed in such a way to bring each bump into contact with the corresponding bump forming point via the adhesive agent, fourth step for fixing the bump on the corresponding bump forming point by solidifying the adhesive agent, and fifth step for releasing the substrate from the bumps fixed on the points on the object (Claim 1, and Page 4, from line 44 in the left column to line 7 in the right column of JP-A No. 9-205095). It has been confirmed that this process can shorten the bump forming step.
JP-A No. 9-10988 proposes a flux material for forming solder bumps, comprising at least a rosin, activator and solvent. More specifically, it comprises a natural rosin and hydrogenated rosin, solute incorporated at an optional content in a range from 1 to 99% by mass relative to the total amount of the natural and hydrogenated rosins, activator working at a predetermined temperature, and solvent for dissolving the natural and hydrogenated rosins, solute, and activator (Claim 1, and from line 13 in the right column in Page 3 to line 18 in the right column in Page 6). The flux material has been confirmed to bring various advantages of, e.g., giving the solder bumps smooth, glossy surface, almost ideal spherical shape and free of voids or cracks inside, and of preventing scattering of solder alloy while the bumps are being formed.
JP-A No. 8-155675 proposes a solder bump forming flux material which brings advantages of giving the bumps of surface gloss, characteristics and shape and of preventing scattering of solder alloy while the bumps are being formed, and comprises at least a rosin, activator containing a component which can be decomposed and sublimated at 100° C. to 300° C. and another component which can be decomposed and activated at 350 to 400° C., and solvent (Claim 1, and line 33 in the right column in Page 4 to line 19 in Page 5).
However, the bump forming process still involves problems, even when it uses such improved flux. It is normally designed to wash out the residual flux on the wafer with various organic solvents, after forming the bump on each pad on the wafer. Therefore, this washing system should secure the safe working environment for the workers, and need some facilities therefor and waste treatment system for safely treating these organic solvent wastes, making the system more complex.
The conventional bump forming process can give the solder bumps whose size is limited to around 50 μm, and is difficult to produce smaller bumps, particularly on a commercial scale, which is one of the basic problems involved in the conventional method, because of the complex steps to be repeated many times. As a result, the bumps formed tend to be uneven, the tendency being more noted as the bump size decreases, to make it more difficult to give the bumps of uniform size. Moreover, the bumps formed by the conventional method tend to suffer voids, cracks and irregular shape, the tendency being also more noted as the bump size decreases.
In other words, the conventional process needs stricter process management as the bump size decreases, and in addition, since the starting materials are mostly wasted for the final product and the process needs heat treatment, it causes environmental and energy efficiency problems.
However, the Internet society will need ICs integrated to a higher extent and mounted at a higher density. Therefore, development of smaller bumps, in particular those having a size of 30 μm or less, have been recently expected. Nevertheless, however, no proposal has been made to meet these requirements.
Moreover, the bumps formed by the conventional method still involve problems, because they are observed to have voids or cracks, and separate from the intermediate metallic layer or pad.
The solder bumps are normally subjected to the following 5 tests.    (1) Metaloscopic observation of outer appearances of solder bumps    (2) SEM observation of bump cross-sections    (3) Mechanical strength tests (shear test/tensile strength test)    (4) Washability test    (5) Reliability test for an assembly with IC chips bonded to a printed-wiring board
The bumps produced by the conventional methods show a number of problems listed below, solutions of which have been strongly demanded.
TABLE 1Testing itemsProblemsSpecified properties or valuesMicroscopic observation (outerBlackened surface, A number ofGloss, Surface characteristics,appearances)bumps of irregular shape orGood shape, Ideally sphericalscattered bumps observed, Anumber of bridges observedSEM observation (outerA number of voids observed, AGood adhesion between the padappearances and cross-section)number of cracks observed, Aand bumpnumber of bumps of irregularshape observed, Excessivediffusion through the BLM filmlayerShear test (strength) 5 gf No Good  >8 gfTensile test500 gf No Good>1200 gfWashability testBlack or white residue observedNo residue observedThermal shock programNo Good in 500 cyclesOK in 1000 cycles(temperature cycle) test30-150° C.Ion migration testNo Good in 500 hoursOK in 1000 hoursHigh-temperature reliability testNo Good in 500 hours at 120° C.OK in 1000 hours at 120° C.,120° C.No Good in 100 hours at 210° C.210° C.210° C.